The present invention relates to an image forming apparatus equipped with a fixing apparatus that fix a toner image on a sheet by heating, pressurizing and by fixing the toner image.
Heretofore, as fixing apparatuses to be used in image forming apparatuses of an electro photographic type such as copying machines, printers, facsimile machines and multifunctional peripherals having function of the aforesaid machines, those of a heating roller type (which is also called heat-fixing roller type) including those covering low speed machines to high speed machines, and those of types of black and white and types of colors, have been widely used. The fixing apparatus of a heating roller type is one wherein a transfer material on which an unfixed toner image is formed is heated and pressurized, by the fixing nip section formed by the heating roller kept to be at a prescribed temperature, and by the pressurizing roller that has an elastic layer and pressure-contacts the heating roller.
A recent demand for color images and for high speed imaging makes a width of a nip to be broader to heat toner on a sheet surface efficiently. In this case, there are considered a way to enlarge two roller diameters or a way to enlarge an amount of distortion (strain) of a roller by enhancing a force of pressure contact between rollers for the purpose of broadening a nip width in the fixing apparatus of the aforesaid heating roller type for its structure. However, when the device of this kind is employed, there are caused problems of a large-sized fixing apparatus and of a lowered durability of the fixing apparatus, resulting in lowered degree of freedom for design conditions.
As a method to solve the problem of this kind, a fixing apparatus of a belt-nip type wherein an endless fixing belt driven by a roller to rotate and a pressure applying member fixed on the inner circumferential surface side are provided, and a fixing belt is pressed against the heating roller by the pressure applying member, has started to be used recently. In the fixing apparatus of a belt-nip type, its structure makes a width of a fixing nip to be broader than that in the heating roller type.
In the fixing apparatus of a belt-nip type, it is structured so that a fixing belt is made to contact the heating roller by the fixed pressure applying member. Therefore, if the sliding resistance in the case of sliding between the inner surface of the fixing belt and the pressure applying member is great, running of the fixing belt is disturbed, resulting in image registration deviation and sheet crease, or a cause for an increase of drive torque.
For the problem of this kind, Unexamined Japanese Patent Application Publication No. H11-045018 and Unexamined Japanese Patent Application Publication No. 2007-079036 disclose a fixing apparatus on which a lubricant supplying member that coats lubricants on an inner circumferential surface of the fixing belt is provided.
Though lubricant is coated on an inner circumferential surface of a fixing belt by a lubricant supplying member in each of Unexamined Japanese Patent Application Publication No. H11-045018 and Unexamined Japanese Patent Application Publication No. 2007-079036, an appropriate amount of coating for lubricant varies depending on the conditions of the fixing apparatus. For example, the conditions of the fixing apparatus are the occasions when the fixing belt, the pressure applying member, the lubricant supplying member and other parts of the fixing apparatus are replaced. Immediately after replacement of the fixing belt with a fresh belt, an inner circumferential surface of the fixing belt has not been coated with lubricant. Therefore, sliding resistance between the pressure applying member and the fixing belt is increased, resulting in the cause of drive torque increase.
When an amount of supplying lubricants is increased, lubricants can be supplied quickly immediately after the replacement, but in the ordinary state, an excessive supply of lubricants is caused, resulting in a problem that an excessive lubricants takes a roundabout route to the surface of the fixing belt to move to the sheet.
In view of the problem mentioned above, an object of the invention is to make an amount of lubricants to be supplied to the fixing belt to be appropriate by changing an amount of lubricants to be supplied depending on the state of the fixing apparatus, and thereby to reduce sliding resistance and to prevent contamination of a sheet by lubricant.